The present invention elates to precipitation hardenable martensitic stainless steel, hereafter called stainless maraging steel. More particularly, the present invention relates to a maraging steel for certain applications, such as in the vehicle industry (cars, trucks, motorcycles for example), where several benefits regarding product properties and manufacturing processes have been obtained.
Normally, carbon steel tubes are used for shock absorbers in cars. These tubes are hardened and surface treated in different ways depending on type of product. The manufacturing process involves many steps and tempering operations, which could cause rejections since the demands regarding dimensional tolerances of such tubes are very high.
It has to be noted that the combination of martensite transformation and precipitation hardening on its If is known from the document Metall. Mater. Trans. A., 25A, 2225-2233, 1994. This document discloses the precipitation in the martensitic structure of intermetallic compounds of quasicrystalline structure based on iron, molybdenum, chromium and silicon. The martensite in said alloy can be formed both by deformation, as described in he document above, or isothermally as described in Scripta Metallurgica et Materialia, 995, Vol.33, No. 9, pp. 1367-1373. This new type of steel alloys was found to exhibit a combination of superior strength, corrosion resistance and ductility. In fact, a tensile strength in the interval 2500-3000 MPa was attained for wire products in the cold worked and aged condition which makes such material well suited for medical and dental instruments. This document does not, however disclose a method of manufacture which allows t form steel products of a desired shape by deformation whilst achieving an optimum between ductility, strength, formability and corrosion resistance and homogeneity of martensite distribution.
The steel alloy treated according to the present invention can be processed in the shape of wire, tube, bar and strip for further use in various vehicle and automotive components. It is an object of the invention to provide a very efficient method for the manufacture of easily formable steel products with a homogeneous distribution of martensite and precipitates making them suitable for usage in components in the vehicle or automotive industry.
By using the stainless maraging steel according to the present invention, the manufacturing process to obtain the final product can be much shorter. Hardening by precipitation of intermetallic compounds gives the product a very high strength level. It is known that material for the application shock absorbers is subjected to very high demands regarding mechanical properties.
In contrast to conventional high strength steels, maraging steels possess certain distinctive characteristics such as lack of distortion during hardening, good weldability and a good combination of strength and toughness that have made them attractive for many applications. In comparison to conventional stainless steels, the physical properties of stainless maraging steels are closer to the properties of the carbon steels used today.
According to a first aspect, the present invention provides a component having corrosion resistance, high strength and toughness, made from a stainless maraging steel material having a composition comprising: at least 0.5% by weight chromium, at least 0.5% by weight molybdenum, the sum of Cr, Ni and Fe exceeds 50% by weight, and the steel has a microstructure including intermetallic particles precipitated into a matrix of martensite.
According to a further aspect, the present invention provides a method of manufacturing a steel alloy component comprising: smelting an alloy having a composition comprising at least 0.5% by weight chromium, at least 0.5% by weight molybdenum, and the sum of Cr, Ni and Fe exceeds 50% by weight; casting the alloy; subjecting the casting to hot extrusion and then to a plurality of cold deforming steps to obtain of at least 50% martensite throughout its microstructure; and subjecting the alloy to an ageing treatment at 425-525xc2x0 C. that is sufficient to obtain precipitation of quasicrystalline particles in the martensitic microstrucure.